1. Field of the Invention
The present invention relates to a wafer aligning apparatus for semiconductor device fabrication, and more particularly, to a wafer aligning apparatus having an anti-static guide roller.
2. Description of the Related Art
Generally, semiconductor devices are manufactured by repeatedly carrying out various kinds of processes such as photolithography, diffusion, etching, ion-implantation and metallization on semiconductor wafers. In order to carry out these processes, the wafers are placed in various kinds of equipment, each designed for a certain processing step.
When the wafers are inserted in such equipment, the wafers often must be aligned according to a certain specific direction because of crystal orientations in the wafer. Production problems can occur if an orientation dependent process is performed on wafers which are not aligned. When the wafers are misaligned in such a process, the process may not be performed accurately, thereby the yield of semiconductor devices produced by the equipment is decreased, and in some cases subsequent processing of the semiconductor device must be discontinued.
Conventional wafer aligning apparatus make use of the flat zone formed on one part of the perimeter of the wafer which is otherwise circular. The flat zone is formed by cutting the edge of a circular wafer (or one side of a cylindrical ingot from which individual wafers are subsequently sliced) along a straight line in a direction that designates one of the standard crystal directions. The conventional apparatus aligns wafers for a certain process by aligning the flat zones of all the wafers in a cassette of wafers undergoing that process.
The conventional wafer aligning apparatus includes a guide roller which contacts the circular edges of the wafers in the cassette with sufficient friction to rotate the wafers. But since the flat zones do not extend as far from the center of the circular portion of the wafer edge, the middles of the flat zones do not contact the guide roller with sufficient friction to rotate the wafers. The wafers are aligned when the guide roller no longer rotates the wafers.
Typically the roller is located below the wafers in the cassette so that gravity helps to maintain frictional contact between the circular, i.e. circumferential, edges of the wafers and the guide roller. In addition, a wafer support placed at the same height as the guide roller holds the flat zones of the wafers out of contact with the guide roller. As a result, all the wafers inside the cassette are aligned with the flat zones along the downward direction within a specific time after the guide roller begins rotating.
However, the conventional wafer aligning apparatus has a problem with static electricity. The conventional guide roller has a shaft that is made of metal, and a roller coat that encloses the shaft and is made of nonconducting synthetic resins, such as rubber or TEFLON. The friction between the wafers and the coat of the guide roller generates static electricity charges that accumulate on the surface of the nonconducting roller coat and on the wafers. If the wafers are immersed in a nonconducting fluid, such as air, the accumulated charges remain on the surfaces of the wafers and roller. Particles around the apparatus with slight charges of opposite sign are attracted to and are also accumulated on the charged wafers and guide roller. These particles contaminate the wafers. In addition, the particles cause corrosion damage on the wafer surface when the accumulated static electricity is suddenly discharged after building to a sufficiently large voltage.
Furthermore, the rubber-made guide roller surface is easily worn down by the friction with the wafers, and produces additional particles that contaminate the wafers.
Since, these friction and the discharge problems have not been solved, there is a need to prevent the generation of static electricity on the guide roller, or to prevent the accumulation of the static electricity and the attracted particles.